Mobile work vehicles are currently equipped with diesel engines and hydrostatic traction. The diesel engines are increasingly being provided with turbochargers which effect a fundamental power increase, but at the same time do not allow the same permissible drag power in drag operation. The design of a hydrostatic transmission is basically known. Work vehicles can be fitted both with one and with a plurality of hydraulic motors, depending on the installed drive power and the requirements placed on maximum traction force and maximum driving speed.
The vehicle fitted with hydrostatic transmission has the advantage that the hydrostatic traction can also act as a brake. This is performed by reducing the pump tilt angle in conjunction with unchanged driving speed. In that instance, the flow of power from the pump to the motor/motors is reversed, and the motors work as a pump. The variable-delivery pump becomes the motor. The energy of the vehicle from mass and speed now drives the units previously designated as hydraulic motors. These units supply volumetric flow to the variable delivery pump. The variable-delivery pump builds up hydraulic resistance, as a result of which the operating pressure (brake pressure) increases. The product of brake pressure x pump swept volume yields a torque which drives the diesel engine in drag operation or in overrun condition at the diesel engine. With very large vehicles, the drag torque to be supported can become so large that the diesel engine substantially overshoots its rated speed, resulting in possible damage. Damage to the remaining components, and loss of hydrostatic braking capacity can result.
It is therefore a principal object of this invention to provide a brake circuit for a mobile work vehicle having a hydrostatic traction system including a closed hydraulic circuit having a variable delivery pump and at least one hydraulic motor, wherein a pressure-limiting valve is in the brake circuit for converting energy from braking energy into heat.
A further object of the invention is to provide a system for controlling a hydraulic vehicle drive in such a way that the possibilities of the hydraulic braking are used to the fullest extent, but non-permissible high revolutions per unit time of the diesel engine driving the variable displacement pump are prevented.
These and other objects will be apparent to those skilled in the art.
A pressure-limiting valve is located in a closed hydraulic circuit of a mobile vehicle having a traction system which includes the brake circuit. The brake circuit also includes a variable-delivery pump and at least one hydraulic motor. The pressure limiting valve is in the brake circuit to convert braking energy into heat. More specifically, this invention is achieved by arranging in one of the lines a pressure regulator that acts as a throttle and that is activated in the presence of an increase in pressure of the liquid flowing from the hydraulic motor to the variable displacement pump. With such an arrangement, as usual, the hydraulic fluid flows at first through the one line, under a higher pressure, toward the hydraulic motor and through the other line, under a lower pressure, back toward the variable displacement pump. However, if the vehicle starts a braking operation, the hydraulic fluid will maintain its direction of flow, but the hydraulic motor now acts as a pump. For this reason, the pressure in the return line rises to a higher value. As soon as it reaches a value which might cause non-permissible high revolutions per unit time of the variable displacement pump and thus of the diesel engine, the pressure regulator of the invention is activated. While the regulator was completely open during normal operation, upon responding to the increase in pressure of the hydraulic fluid returning from the hydraulic motor, the regulator acts as a flow throttle, for example, in the form of a shield. At that point, a specific energy conversion of flow energy into heat takes place and the braking energy is reduced appropriately. The arrangement of the invention can be fine-tuned in such a way that a maximum hydraulic braking action is combined with revolutions per unit time which are not dangerous to the diesel engine.
In practice, the vehicles are designed for high transfer speeds, i.e., for fast driving in the forward direction. Thus, they can drive faster in the forward direction than in the reverse direction. The indicated problems occur specifically during braking while driving at fast speeds in the forward direction. For this reason, the arrangement of the invention is important particularly when it becomes effective while driving in the forward direction.
In accordance with a first refinement, however, it is also possible to provide a pressure regulator in each of the two lines so that the arrangement of the invention finds application during forward and reverse driving.
The pressure regulator, which is arranged in the line, provides flow resistance when the hydraulic fluid flows through this line in the opposite direction, i.e., the line serves as a supply line for the hydraulic motor. In order to eliminate this disadvantage, in accordance with a further development, a return valve is provided parallel to the pressure regulator and which opens in the direction of supply of hydraulic fluid to the hydraulic motor. Thus, in case of driving in the opposite direction, the supply of hydraulic fluid to the hydraulic motor can take place despite the presence of the pressure regulator without additional flow resistances.
The drive of the pressure regulator is possible in all known ways, for example, electrically or hydraulically.
In accordance with an advantageous refinement of the invention, by skillfully fine-tuning the entire arrangement, a certain operating pressure of the hydraulic fluid in the closed cycle can be built up at the variable displacement pump which deviates from the operating pressure at the hydraulic motor. This operating pressure is then available to the vehicle as an exactly defined braking pressure.